The invention concerns an apparatus for processing physiological signals, which includes a signal pick-up unit for picking up physiological measurement signals, in particular for picking up electrical signals of a heart, and first detection means which are connected to the signal pick-up unit and include signal processing means such as a filter and a first threshold value store and a comparison unit which is connected thereto and which is adapted to output a first marker signal if the amplitude of the measurement signal exceeds a threshold value stored in the first threshold value store, and second detection means which are connected to the signal pick-up unit and include a second threshold value store and a second comparison unit which is connected thereto and which is adapted to output a second marker signal if the amplitude of the measurement signal exceeds a threshold value stored in the second threshold value store.
Signal processing apparatuses of that kind can for example be a component part of a cardiac pacemaker, a cardioverter and/or a defibrillator. A specific purpose of such signal processing apparatuses is the detection of chamber fibrillation phenomena, in particular the detection of ventricular fibrillation, which require anti-tachycardia therapy or cardioversion.
In particular the detection of ventricular fibrillation phenomena, that is to say physiologically disproportionately high heart rates involves some problems. It is known in principle for the heart rates to be determined by reference to an electrocardiogram by a procedure whereby the R-spikes of the QRS-complexes contained in the electrocardiogram are detected and the frequency at which the R-spikes occur, the RR-rate, is determined. A problem in that respect involves in particular a greatly fluctuating amplitude of the R-spikes of an electrocardiogram and the differing width of the QRS-complexes.
The state of the art discloses various apparatuses for the detection of ventricular fibrillation. U.S. Pat. No. 4,393,877, to Imran, teaches providing two input channels for detecting measurement signals, which involve a differing level of sensitivity for the rise gradient of ECG-signals and linking signal features of the two channels, which are recognised by way of the rise gradient, by way of an xe2x80x98exclusive-orxe2x80x99 function. Baker, in U.S. Pat. No. 4,880,004, describes a heart stimulator which has two input channels with differing band pass characteristics, of which one serves for the detection of cardiac events and the second serves for automatic adaptation of the level of detection sensitivity of the first input channel.
White, in U.S. Pat. No. 5,562,709, teaches an atrial defibrillator having two input channels for R-spike detection, which have different band pass characteristics and which operate with respectively specific amplitude threshold values for R-spike detection. The detection results of the two input channels are logically xe2x80x98andxe2x80x99-linked for producing a synchronisation signal. For resetting an interval timer the detection results of the two input channels are logically xe2x80x98orxe2x80x99 linked. A third separate input channel is provided for the detection of atrial fibrillation.
It has been found that the detection in particular of ventricular fibrillation, which is possible by means of the known apparatuses, leaves something to be desired in particular in terms of reliability.
Therefore the object of the present invention is to provide an apparatus for processing physiological signals, which with means of the utmost simplicity permits reliable detection of events contained in particular in cardiac signals and/or given states of the heart such as the presence of ventricular fibrillation.
In accordance with the invention that object is attained by means of an apparatus of the kind set forth in the opening part of this specification, including an evaluation unit which is connected to the first and second detection means for taking over a respective series of the first and second marker signals and for processing the series of marker signals to afford a single series of combined marker signals, for analysis of the time succession of the marker signals, in particular the intervals between marker signals of the combined series, and output of a recognition signal which is dependent on the analysis.
In the context of the invention, the recognition signal is a signal which identifies a state of the heart that is to be detected, and not an event such as an R-spike as such. Analysis of the combined series is thus not directed to the respective individual events such as R-spikes, but to the time succession of events, which is represented by the marker signals, in particular the intervals between the marker signals as a reproduction, which is already filtered, of the events. That analysis is aimed at the detection of such states as fibrillation. An apparatus of that kind advantageously makes it possible to determine RR-intervals and thus heart rates by analysis of the intervals between marker signals of a combined series of marker signals, wherein the combined series contains marker signals which originate from input channels having different detection properties. In particular an apparatus of that kind makes it possible on the one hand for the parameters of the detection means, which are crucial in terms of the detection of events and output of corresponding marker signals, such as input sensitivity, frequency characteristic and magnitude of the threshold value, and on the other hand the manner and way in which the series of marker signals obtained in two different ways are combined to form a series by suitable means to be so selected and matched to each other that the recognition of heart rates and thus also fibrillation recognition can be reliably implemented.
For analysis of the combined series of marker signals, the evaluation unit may include a counter that counts the marker signals or the intervals between marker signals within a predetermined portion of the filtered series of marker signals. That alone already makes it possible to determine the heart rate for the respective measurement signal portion. In combination with analysis of the interval lengths within the respective portion of the combined series of marker signals, it is possible to form a reliable criterion in regard to the presence of fibrillation, as is described in greater detail hereinafter.
A typical analysis is one for which a given number of for example 48 intervals is counted off and a check is made to ascertain how many of those 48 intervals are shorter than a comparison interval. A recognition signal indicating fibrillation is outputted when 36 of the 48 compared intervals are shorter than the comparison interval. In that case the evaluation unit includes for example means for counting a total number of intervals and for counting those intervals within that total number of intervals, which are shorter than a comparison interval. In addition the evaluation unit includes means for comparing the counted values to corresponding comparison values and means for outputting the recognition signal in dependence on the comparison result.
In many embodiments of the apparatus, the first selection means includes a first absolute value unit which is adapted to form the absolute value of the measurement signal. That absolute value unit may be connected upstream of the first comparison unit. In relation to the measurement signal, this means that negative measurement signal values are used with inverted signs for the threshold value comparison operation while positive measurement signal values retain their sign. It has been found that this markedly improves the reliability of fibrillation recognition.
In many embodiments, the second detection means also includes a second absolute value unit that is connected upstream of the second comparison unit.
In addition, many embodiments will be provided with a high pass filter for the measurement signal, which is either already arranged in the signal pick-up unit or which is associated with the second detection means so that the signal to be processed by the second detection means is subjected to high pass filtering with a limit frequency which is typically of an order of magnitude of 10 Hz. In that way, high-frequency interference signals are suppressed, without the band width of the signal to be processed by the second detection means being excessively greatly restricted.
Some embodiments of the second detection means have a high pass filter with a limit frequency which is typically of an order of magnitude of between 20 and 30 Hz and is thus above that of the high pass filter in the signal pick-up unit or the first detection means. The signal to be processed by the second detection means is thus frequently-limited to a greater degree than the signal to be processed by the first detection means. Such measurement signal components at a frequency of between 10 and 20 or 30 Hz are thus only processed by the first detection means.
In addition the first detection means will often have a noise limiting unit that suppresses further processing of the measurement signal within a predeterminable noise suppression time in accordance with those signal values of the measurement signal, which result in a first marker signal. In that respect each such signal value freshly triggers the first noise suppression period. The first noise suppression unit is thus retriggerable. The first noise suppression period is typically in the order of magnitude of 125 ms. The first noise suppression period is usually triggered by the rising signal edge of that signal value which exceeds the first threshold value and thus results in a first marker signal. An edge detector is appropriately provided for detection of the rising signal edge.
Many embodiments of the second detection means will have a noise limiting unit for triggering a second noise suppression period that is triggered by the rising edge of such a signal value that exceeds the second threshold value. The second detection means will also often have an edge detector. The triggered noise suppression period is retriggerable, that is to say, it is freshly triggered by each signal value which exceeds the threshold value and which also occurs within the noise suppression period. The second noise suppression period is of an order of magnitude of about 50 ms.
A typical embodiment of the first detection means will also have a refractory timer which triggers a first refractory time if a measurement signal value exceeds the first threshold value and thus triggers a first marker signal if that occurs outside the first refractory time. The refractory timer is thus not retriggerable and responds only to signals which are outside the refractory time. The first refractory time is composed of an absolute refractory time of typically 150 ms and a relative refractory time, the total refractory time typically being 300 ms. Events occurring throughout the entire refractory time of 300 ms, after detection of a signal value which exceeds the threshold value, are not taken into consideration in terms of further processing. The first refractory timer is connected downstream of the first noise limiting unit.
Some embodiments of the second detection means will also include a second non-retriggerable refractory timer for a second refractory time. The second refractory time is about 135 ms.
The edge detectors for detecting rising signal edges and triggering the first or second noise suppression period or the first or second refractory time respectively are commonly so designed that they are responsive to a threshold value-passage of the measurement signal in the positive direction.
An essential feature of the apparatus according to the invention is the evaluation unit. That is adapted to take over the series of first and second marker signals, which originate from the first and second detection means, more specifically in such a way that the time relationship between the first and second marker signals, which is given by way of the measurement signal and its signal values which exceed the threshold value and which lead to marker signals, is maintained. Taking over the series of first and second marker signals from the first and second detection means is thus effected in mutual time correlation or in mutually synchronised relationship.
In at least one alternative embodiment, the evaluation unit includes a logic unit by which the series of first and second marker signals are combined in the manner of xe2x80x98orxe2x80x99 linking to form a common series, wherein first and second marker signals which are within a time window of about +/xe2x88x9250 ms are combined to afford a marker signal of the combined series. That combined series will usually be filtered by a logic filter unit in such a way that the filtered series of marker signals contains only such marker signals which lie outside a masking time in the range of about 300 ms which is triggered by each marker signal of the second series of marker signals, which comes from the second detection means.
In at least one embodiment, the evaluation unit includes:
coincidence recognition means which are adapted to output a coincidence signal when one of the first marker signals falls outside the first refractory time and one of the second marker signals falls outside the second noise suppression period in a coincidence period of about 50 ms,
masking means which are connected to the coincidence recognition means and which are adapted to mask those of the second marker signals for further signal processing, which within a first masking time of about 300 ms follow a second marker signal which led to a coincidence signal,
coincidence transfer means which are connected to the coincidence recognition means and which are adapted to add the first of the first marker signals which follows a coincidence signal outside the first noise suppression period and within a transfer period of about 300 ms, while retaining the time relationship of the series of the second marker signals, and thus to form a modified series of marker signals, and
coincidence transfer masking means which are connected to the coincidence transfer means and which are adapted to mask out of the modified series of marker signals all those marker signals which within a second masking time of about 300 ms follow a marker signal added by the coincidence transfer means, in order thereby to form a filtered series of marker signals.
With an evaluation unit of that kind, a series of rules is converted into practice, resulting in an optimised combined series for further analysis.
The two alternative configurations of the evaluation unit may additionally include:
interval determining means which are adapted to determine the interval duration of time intervals between two adjacent marker signals of the filtered series of marker signals,
a comparison interval store for a comparison interval time duration,
interval comparison means which are connected to the interval determining means and the comparison interval store and are adapted to detect those intervals of the filtered series of marker signals, which are shorter than the comparison interval time duration,
a counter which is connected to the interval comparison means and adapted to determine the number of marker signals within a portion of the filtered series of marker signals or intervals between two respective adjacent marker signals and the number of those intervals which are shorter than the comparison interval time duration, and
a signal generator which is connected to the interval counting unit and a quotient store and is adapted such that the signal generator outputs a signal if the proportion of the intervals which are shorter than the comparison interval time duration exceeds a proportion, stored in the quotient store, of the total number of intervals of a portion of the filtered series.
By virtue of those means, the evaluation unit is capable of counting off a given number of intervals and determining how many of those intervals are shorter than a comparison interval. In specific terms, the evaluation unit can be so designed that the counter counts off a predetermined number of intervals, for example 48, and, by means of the interval comparison means, marks those intervals which are shorter than the comparison interval. If the number of marked intervals within the 48 intervals exceeds for example the value 36, a signal characterising fibrillation is outputted. If therefore the counter is so designed as to count in each case 48 intervals, then the signal generator can include a second counter for the intervals which are shorter than the comparison interval. The quotient store then stores a limit value for the number of those shorter intervals and the signal is triggered if the value in the second counter exceeds the value stored in the quotient store. In other words, if the denominator of the quotient, for example 48, is predetermined by the first counter, the quotient store of the signal generator only has to include the numerator of the quotient, for example 36. Other per se known means which provide that the evaluation unit outputs a signal if out of for example 48 last-detected intervals more than 36 are shorter than a comparison interval, are also suitable for implementation of alternative configurations of the evaluation unit.
Some embodiments of the signal pick-up unit include a decimator that samples the measurement signal at a first rate and outputs sample values for the measurement signal at a second rate, with the first rate typically eight times as fast. That permits sampling of the measurement signal with an adequate degree of accuracy and results in a first data reduction which significantly restricts complication and expenditure in terms of further processing of the measurement signal.
Many embodiments of the signal pick-up unit also include an analog/digital converter for the measurement signal so that the measurement signal is converted into a series of time-discrete, digital values which can be subjected to further digital processing.
Some signal pick-up units will include averaging means for forming a sliding average of the measurement signal. That provides for suppressing dc signal components of the measurement signal, which are possibly subjected to drift, prior to further processing by the two detection means.
Some first detection means will have a signal amplifier for the measurement signal with an adaptable gain factor. In the case of a heart stimulator the gain factor is should be increased if the stimulator delivers a stimulation pulse. In addition or as an alternative the gain factor is also increased if the second detection means have detected, within 200 ms prior to delivery of the stimulation pulse, a signal value which exceeds the second threshold value and thus results in a second marker signal.
The second detection means should be designed to be such that they can be switched off and are activated by an activation unit if the gain factor of the signal amplifier of the first detection means is increased.
Alternatively or in addition the activation unit also activates the second detection means if the signals detected by the first detection means exceed a frequency of 140 signals per minute.
The second detection means can be designed for time-shifted processing of the measurement signal values and for that purpose often includes a signal store for the digitised measurement signal which is outputted by the averaging means. That signal store can be in the form of a ring store or first-in, first-out store (FIFO-store).